1. Field of The Invention
This invention relates to communications systems, and more particularly to a system for reducing the peaks of a signal to be amplified.
2. Description of Related Art
An ideal power amplifier amplifies an input signal of a given bandwidth with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, a power amplifier, however, has a transfer function with nonlinear and xe2x80x9clinearxe2x80x9d regions. Whether the power amplifier is operating in a linear or nonlinear region depends on the amplitude of the input signal. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal peak amplitude causes the amplifier to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the amplifier is being overdriven, and the output signal is clipped or distorted. Generally, an amplifier is characterized as having a clipping threshold, and input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output. In addition to distorting the signal, the clipping or nonlinear distortion of the input signal causes power to be generated in adjacent channels or frequencies to corrupt or interfere with signals in the adjacent channels or frequencies, commonly referred to as spectral regrowth or adjacent channel power (ACP). The generation of adjacent channel power is of particular concern in wireless communications systems where signals being amplified are in adjacent channels or frequency bands.
In communication systems where signals have a large range of amplitudes, components such as power amplifiers have to maintain linearity over a large dynamic range to avoid generating nonlinear distortion of the input signal and spectral adjacent channel power. For example in signals transmitted in the standard identified as EIA/TIA/IS-95(Electronic Industries Association/Telecommunications Industry Association/Interim Standard 95) entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, Mar. 1993 (xe2x80x9cIS-95xe2x80x9d), the peak to average power ratio (PAR) for a single loaded code division multiple access (CDMA) 1.25 MHz carrier is about 11.3 dB measured at 10xe2x88x924 peaking probability (indicating a 1/10,000 chance that a peak exceeds a threshold value for example of 8.5 dB above the average power). As such, a high cost, low efficiency power amplifier is operated at an average power level far below its saturation point to avoid distorting the peaks and generating ACP. To reduce the cost and improve the efficiency of the power amplifier and other components, methods have been proposed for reducing the PAR of the signal without generating significant ACP or spectral regrowth which occurs when power from the signal appears at different frequency bands.
A straight forward method for PAR reduction involves hardlimiting or hard clipping the signal peaks to a certain hardlimiting threshold to lower the PAR. A hard clipped signal, however, has abrupt edges and the short time duration of the clipped edges generates significant adjacent channel power and spectral regrowth in the frequency domain. A filter can be used to remove the ACP and the spectral regrowth. For a radio transmitter, the filter can be implemented at baseband or intermediate frequency (IF) where sharp filters are readily available either in digital or analog form. A digital implementation at baseband is favored because of its flexibility and low cost. It has been found, however, that after filtering the clipped signal, the signal peaks grow back. Accordingly, the peaks could be detected using the desired threshold, then the peaks above the detection threshold are hardlimited to a new limit lower than the detection threshold. When the peaks grow back after the cleanup filtering, they would still be below the targeted value. For digital hardware implementation, this method is as easy to implement as the simple hardlimiting. However, the peaks that are slightly above the threshold only needs to be clipped lightly and their growth after cleanup filtering is also small. By clipping all these peaks to the new limit, the peaks that are slightly above the threshold can be over-clipped.
U.S. Pat. No. 5,287,387 discloses a window clipping method for reducing the peak to average power ratio of an input signal. In the window clipping process, an attenuating window is centered about a local maximum of a signal peak above a threshold, and the attenuating window is multiplied with the input signal to generate an attenuated signal. The attenuating window is comprised of multiple sample weights which are valued at less than one. The multiple weights of the attenuating window are multiplied with corresponding peak samples of the input signal to reduce the peak of the input signal to below a threshold. However, multiplying the signals in the time domain is equivalent to convolving the spectrum of the input signal with the window spectrum in the frequency domain, thereby altering the spectrum of the window clipped signal. Thus, the window clipping process fails to adequately address the problems of the hard clipping processes.
The present invention reduces signal peaks by notching the peak of a signal above a threshold to produce a notched signal. The notched signal is then filtered to produce a resulting signal with a reduced peak amplitude. For example, in an implementation where the signal is represented by signal samples, the peak notching system first locates a peak sample that is beyond a threshold, such as a sample representing a positive peak sample of a peak above the threshold. Once a peak sample is located, the peak notching system adjusts the peak sample by an amount which is a function of the amount that the peak sample is beyond the threshold, effectively creating a notched signal with a one sample notch at the peak. The peak notching system filters the notched signal to fill in the notch to produce a signal with a reduced peak.